When using a chain saw there is a risk of several kinds of severe accidents.
There might be so-called kickback accidents, i.e. the saw bar with its rotating saw chain is swung up towards the saw operator. There might be fall accidents, when the user perhaps slips and falls over the saw with its rotating saw chain. In order to reduce the risk of damages at the event of kickback the chain saw is normally equipped with a chain brake that is actuated by a so-called kickback guard. This is mounted in front of the chain saw's front handle, a so-called handle frame. At the event of kickback the user's forehand often hits the kickback guard, which then actuates the chain brake. The chain brake can also be actuated due to the inertia of the kickback guard when the rapid kickback occurs. However, there might be accidents due to slow kickbacks, or when the user falls over the saw, where it might be a great risk that the above-mentioned trigger principles would not function.
PCT Application Publication No. WO 86/04294 (which corresponds to Swedish Patent No. 441992) describes a trigger device for a kickback guard that is so designed that it can be actuated by the rear hand. A trigger handle is located above the rear hand and is entirely mechanically connected to the chain saws ordinary kickback guard located in front of the front handle frame. The trigger handle is thus connected to the engine unit that is not equipped with vibration damping, while the rear handle as well as the handle frame are isolated from vibrations. This is a clear disadvantage. A redesigned version has been produced and marketed, where a trigger handle is mounted to the rear handle and connected via a wire to a trigger mechanism for the chain brake. The trigger arm is designed as a lever journalled a few centimeters above the saw's throttle control. This location of the journal results in that the angle of the operator's arm will not correspond particularly well with the angle of the trigger arm so that the trigger arm will chafe against the arm, both at easy contact or at strong contact, i.e. at an actual release. Furthermore, the trigger wire in this design would have to be drawn totally unprotected at the outside of the saw over a great deal of its length. Hereby there is a risk that it might be damaged.